It is known to remove flash from molded plastic and elastomeric articles by chilling the workpiece to embrittle the flash in order to facilitate its removal by impact with a high velocity stream of solid granular media in the form of shot or pellets.
In a typical operation the piece or pieces to be treated are introduced into a heat-insulated chamber maintained at required low temperature and the stream of blasting media is impelled at high velocity against the surface of each piece by one or more rotating impellers or so-called throwing wheels. The discharged blasting media together with the fragments of the flash thereby removed, are collected and conveyed out of the treating chamber to a screening apparatus in which the blasting media is separated and recovered for recycling to the blasting operation; and the refuse comprising larger fragments of removed material as well as fines are discharged. Several embodiments of such systems are described in the above-cited pending application Ser. No. 445,778, and in the earlier prior art cited in said pending application which disclosure is incorporated herein by reference. The preferred blasting media advocated is pelleted polycarbonate resin.
As indicated in the above-cited pending patent applications and the prior art therein listed, the removal of flash and coatings from articles by blasting of the chilled articles may be carried out by batch or continuous type processes. In the continuous type process, with which the present patent application is more particularly concerned, the material to be treated is moved by a conveyor through an elongated tunnel and during such passage it is subjected to chilling and contact with the blasting media. Systems of the continuous type are described, for example, in U.S. Pat. Nos. 3,824,739 and 4,312,156, and in the embodiments illustrated by FIGS. 6 and 7 of said pending application Ser. No. 445,778.
As described with respect to the continuous mode embodiment in said application Ser. No. 445,778, the articles to be subjected to impact by the blasting media are passed through the treating tunnel on a foraminous endless belt. Liquefied gas, such as liquid nitrogen (LIN) is directed downwardly toward the articles on the moving belt, through spray nozzles located ahead of the first of two throwing wheels mounted on the roof of the tunnel and arranged to impel the blasting media downwardly into contact with such articles.
During initial full-scale production operation of a cryogenic deflashing system wherein liquid nitrogen employed as the chilling medium was sprayed in liquid form directly on the molded articles, several problems were encountered. It was found that in spraying of the cold liquid, particularly on the articles such as padded automobile arm rests, a considerable number of the treated articles were cracked at the surface. Another major problem heretofore encountered was the sticking of blasting media to the undersides of the articles subjected to deflashing, found to be due to the mold release wax present thereon. To remove the sticking media particles it was necessary to employ an additional operator to blow off the media with compressed air.